[winswitch] multifile format

[Mukul Agrawal]

If I want xpra proxy on machine1 to connect to xpra server on machine2 using ssh with public key authentication and no password, then how should I set it up? 
Public keys are already in default locations and xpra is able to attach directly from machine2 to machine 1 using standard format:  xpra attach ssh:username at machine1:display.
But when I try to connect via proxy from client machine3, proxy is not being able to authenticate. It sends the challenge but then there is no log after that.

I am using multifile like this :-
username|pswd|1000|1000|ssh:machine1:display|| 
and attach command from machine3 like this:-xpra attach tcp:username:pswd at machine2:proxyPORT

Are the usernames and passwords actually associated with login accounts on the target machine or their significance is only with respect to multifile resolution?
Can password be left blank for public key authentication?

Thanks.